Effects of river capture and sediment flux on the evolution of plateaus: Insights from numerical modeling and river profile analysis in the upper blue nile catchment

Abstract

The Ethiopian Highlands, with up to 1,500-m-deep canyons surrounded by low relief plateau surfaces, are one of the most spectacular examples of transient fluvial landscapes on Earth. We analyze river profiles extracted from a 90 mdigital elevation model of the upper Blue Nile catchment and identify 116 major knickpoints on 137 river profiles. We use 1-D river profile models to simulate three potential mechanisms for knickpoint formation: plateau uplift, capture of large lakes or internal drainage on the plateau surface, and sediment-flux-dependent river incision with a low sediment flux from the plateau surface. We define a normalized upstream knickpoint propagation distance, χkpj, and demonstrate that the erosion models predict different distributions of this metric in transient profiles following plateau uplift. Model knickpoints resulting from the scenario of plateau uplift or common base-level fall using the stream-power model display similar upstream propagation distance in χ space. The results of the same scenario modeled with the sediment-flux-dependent incision model show upstream knickpoint propagation distance proportional to catchment area. Perturbations to these trends result from drainage capture. Comparing the model results with observed χkpj values of knickpoints and field observations, we recognize effects characteristic of the sediment-flux-dependent incision model. However, most profiles are best explained by the systematic transfer of drainage area from the plateau to the surrounding rivers. We propose a new model of landscape evolution for the upper Blue Nile catchment dominated by discrete events of capture of drainage area from the plateau.